1. Field of the Invention
The present invention relates to a transflective liquid crystal device (hereinafter also referred to as a transflective liquid crystal device) capable of both reflective display, which reflects incident light to display an image, and transmissive display, which transmits incident light to display an image.
2. Description of the Related Art
The transflective liquid crystal device is widely used as a display device of portable information equipment. FIG. 11 schematically illustrates the structure of a conventional transflective liquid crystal device 1000. The transflective liquid crystal device 1000 includes an absorptive polarizer 1020, a liquid crystal cell 1030, a light diffusing plate 1040, a reflective polarizer 1050, and a light absorbing plate 1060. A backlight 1070 is further disposed outside the light absorbing plate 1060. The liquid crystal cell 1030 includes a lower glass substrate 1033, an upper glass substrate 1031, and a liquid crystal layer 1035 sealed between these glass substrates 1031 and 1033. A plurality of transparent signal electrodes 1034 are mounted on the upper surface of the lower glass substrate 1033. A plurality of transparent scanning electrodes 1032 are mounted to be perpendicular to the plurality of signal electrodes 1034 on the lower surface of the upper glass substrate 1031. The liquid crystal cell 1030 has a passive matrix configuration, in which one pixel is defined by one signal electrode 1034, one scanning electrode 1032, and the liquid crystal layer 1035 between these electrodes 1034 and 1032. Namely the light transmitted through the liquid crystal layer 1035 is modulated according to the voltage applied between one signal electrode 1034 and one scanning electrode 1032. The liquid crystal layer 1035 may be made of a TN (twisted nematic) liquid crystal composition or STN (super twisted nematic) liquid crystal composition. A translucent film having the transmittance of about 50% is used for the light absorbing plate 1060.
FIG. 12 shows problems arising in the conventional transflective liquid crystal device 1000. The absorptive polarizer 1020 has an axis of transmission 1020T that is set parallel to the plane of the drawing, and an axis of absorption 1020A that is perpendicular to the plane of the drawing. The reflective polarizer 1050 has, on the other hand, an axis of transmission 1050T that is parallel to the plane of the drawing, and an axis of reflection 1050R that is perpendicular to the plane of the drawing. The following describes the operations of the liquid crystal display 1000 on the assumption that the polarizing direction of the light transmitted through the liquid crystal cell 1030 is rotated by 90 degrees while no voltage is applied between the signal electrodes 1034 and the scanning electrodes 1032 (that is, when the liquid crystal cell 1030 is in an OFF state).
This liquid crystal device 1000 has two display modes, that is, a reflective display mode using incident light 1100 from the outside and a transmissive display mode using light 1120 emitted from the backlight 1070. In the reflective display mode, when the non-polarized light 1100 enters the absorptive polarizer 1020, a linearly polarized light component having the polarization direction parallel to the axis of absorption 1020A is mostly absorbed by the absorptive polarizer 1020, while only a linearly polarized light component having the polarization direction parallel to the axis of transmission 1020T is transmitted through the absorptive polarizer 1020 and enters the liquid crystal cell 1030. The optical rotatory power of the liquid crystal cell 1030 causes the light component entering the liquid crystal cell 1030 to be converted into linearly polarized light having a polarizing direction that is perpendicular to that of the incident light. The polarizing direction of the light emitted from the liquid crystal cell 1030 is substantially identical with the direction of the axis of reflection 1050R of the reflective polarizer 1050, so that most of the light emitted from the liquid crystal cell 1030 is reflected by the reflective polarizer 1050 and re-enters the liquid crystal cell 1030 as return light. The liquid crystal cell 1030 converts the return light into linearly polarized light having a polarizing direction that is perpendicular to that of the return light. At this moment, the polarizing direction of the return light emitted from the liquid crystal cell 1030 is substantially identical with the direction of the axis of transmission 1020T of the absorptive polarizer 1020, so that most of the return light emitted from the liquid crystal cell 1030 is transmitted through the absorptive polarizer 1020. In the reflective display mode, the pixels where the liquid crystal cell 1030 is in the OFF state receive the light reflected and returned as discussed above and are thereby observed as bright pixels. The pixels where the liquid crystal cell 1030 is in an ON state are, on the contrary, observed as dark pixels.
In the transmissive display mode, on the other hand, when the non-polarized light 1120 enters the reflective polarizer 1050, a linearly polarized light component having the polarization direction parallel to the axis of reflection 1050R is mostly reflected by the reflective polarizer 1050, while only a linearly polarized light component having the polarization direction parallel to the axis of transmission 1050T is transmitted through the reflective polarizer 1050 and enters the liquid crystal cell 1030. The optical rotatory power of the liquid crystal cell 1030 causes polarizing direction of the light transmitted through the liquid crystal cell 1030 to be converted into a direction substantially parallel to the axis of absorption 1020A of the absorptive polarizer 1020. Most of the light emitted from the liquid crystal cell 1030 is accordingly absorbed by the absorptive polarizer 1020 and is not transmitted through the absorptive polarizer 1020. In the transmissive display mode, since the light is absorbed in the course of the optical path, the pixels where the liquid crystal cell 1030 is in the OFF state are observed as dark pixels. The pixels where the liquid crystal cell 1030 is in the ON state are, on the contrary observed as bright pixels. The relationship between the ON/OFF state of the liquid crystal cell 1030 and the bright/dark state of the pixel in the transmissive display mode is reverse to that in the reflective display mode. In the transflective liquid crystal device 1000, the brightness and darkness of display are reversed between the reflective display mode and the transmissive display mode.
The object of the present invention is thus to provide a liquid crystal device that effectively prevents the reversion of the same bright/dark states between the reflective display mode and the transmissive display mode, and also to provide an electronic apparatus using such a liquid crystal device.
At least part of the above and the other related objects is attained by a liquid crystal device that modulates light responsive to given image signals. The liquid crystal device includes a first absorptive polarizer, which receives light from outside;a liquid crystal cell, which receives light emitted from the first absorptive polarizer;a second absorptive polarizer, which receives light emitted from the liquid crystal cell; and a reflective polarizer, which receives light emitted from the second absorptive polarizer. The reflective polarizer has an axis of reflection in a predetermined direction to reflect at least part of light that has been transmitted through the first absorptive polarizer, the liquid crystal cell, and the second absorptive polarizer to be incident on the reflective polarizer. The reflective polarizer partially transmits light including a linearly polarized light component which is included in light entering the reflective polarizer from an opposite side to the second absorptive polarizer and which is to be transmitted through the second absorptive polarizer. The first absorptive polarizer has an axis of transmission in a specific direction to cause light, which has been reflected by the reflective polarizer and transmitted through the second absorptive polarizer, to be transmitted through the first absorptive polarizer.
The liquid crystal device of the present invention works as discussed below in a first state of the liquid crystal cell, in which the light entering the first absorptive polarizer from the outside is transmitted through the first absorptive polarizer, the liquid crystal cell, and the second absorptive polarizer. Part of the light that is emitted from the second absorptive polarizer and includes a linearly polarized light component having a polarizing direction parallel to the axis of reflection of the reflective polarizer is reflected by the reflective polarizer, transmitted through the second absorptive polarizer and the liquid crystal cell, and emitted from the first absorptive polarizer. When the light enters the reflective polarizer on the opposite side to the second absorptive polarizer in the first state of the liquid crystal cell, on the other hand, part of the light is transmitted through the reflective polarizer, the second absorptive polarizer, and the liquid crystal cell in this sequence and emitted from the first absorptive polarizer. In this first state of the liquid crystal cell, the light entering the first absorptive polarizer from the outside is reflected and emitted to the outside. The light entering the reflective polarizer is also eventually emitted to the outside. The liquid crystal cell in the first state is accordingly observed as a bright pixel both in the reflective display mode and in the transmissive display mode.
The liquid crystal device works as discussed below in a second state of the liquid crystal cell, in which the light entering the first absorptive polarizer from the outside is absorbed by the second absorptive polarizer. The light supplied from the outside into the first absorptive polarizer is absorbed by the second absorptive polarizer, so that there is no light reflected by the reflective polarizer. Namely the light entering the first absorptive polarizer from the outside is not emitted from the first absorptive polarizer. When the light enters the reflective polarizer on the opposite side to the second absorptive polarizer in the second state of the liquid crystal cell, on the other hand, part of the light is transmitted through the reflective polarizer, the second absorptive polarizer, and the liquid crystal cell in this order and enters the first absorptive polarizer. This light is, however, absorbed by the first absorptive polarizer and is thereby not emitted. In this second state of the liquid crystal cell, the light entering the first absorptive polarizer from the outside is not emitted to the outside. The light entering the reflective polarizer is nor emitted to the outside. The liquid crystal cell in the second state is accordingly observed as a dark pixel both in the reflective display mode and in the transmissive display mode.
As discussed above, the liquid crystal device of the present invention effectively maintains the same bright/dark states in both of the reflective and transmissive display modes.
In accordance with one preferable application, the liquid crystal device further includes a diffusing plate interposed between the second absorptive polarizer and the reflective polarizer.
This arrangement effectively suppresses specular reflection occurring on the reflective polarizer.
In accordance with another preferable application of the liquid crystal device, the predetermined direction of the reflection axis of the reflective polarizer is adjusted to cause a ratio of an amount of first light to an amount of second light to be not less than about 15% in a state where a linearly polarized light component having a predetermined first polarizing direction is emitted in a greatest amount from the liquid crystal cell towards the second absorptive polarizer. The first light is one that is reflected by the reflective polarizer and transmitted through the second absorptive polarizer, the liquid crystal cell, and the first absorptive polarizer. The second light is one that is incident on the first absorptive polarizer.
Unlike the conventional liquid crystal display, this arrangement enables non-reversed transmissive display without unduly affecting the advantageous characteristics (including brightness) of reflective display.
In accordance with still another preferable application, the liquid crystal device further includes a backlight disposed opposite to the second absorptive polarizer across the reflective polarizer. Light emitted from the backlight is adjusted to have a color other than white, in order to cause color of a first light to be close to color of a second light. The first light is one that is emitted from the backlight and transmitted through the reflective polarizer, the second absorptive polarizer, the liquid crystal cell, and the first absorptive polarizer. The second light is one that comes from the outside and is transmitted through the first absorptive polarizer, the liquid crystal cell, and the second absorptive polarizer, subsequently reflected by the reflective polarizer, then transmitted through the second absorptive polarizer, the liquid crystal cell, and the first absorptive polarizer
In this structure, it is preferable that the backlight includes a light source and a color filter that adjusts color of light emitted from the light source.
This arrangement enables the color of a transmitted light component that is included in the light emitted from the backlight, transmitted through the reflective polarizer, and emitted from the first absorptive polarizer to be adjusted close to the color of a reflected light component that is supplied from the outside to the first absorptive polarizer, reflected by the reflective polarizer, and emitted from the first absorptive polarizer. This reduces a difference in color tone of the display between the reflective display mode and the transmissive display mode.
Any one of the above liquid crystal devices may be mounted as a display device on a variety of electronic apparatuses.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.